Prior to the present invention, various control concepts are known and are in use for anti-locking brake systems which are arranged in series. Included in the prior art are various regulation concepts for anti-lock brake systems that are in mass production.
For example, the single wheel or individual control and regulation (IC) anti-lock brake system envisions each wheel on the vehicle having a separate control circuit including its own sensor and setup unit or actuator. Such setup unit or actuator is a solenoid valve. This system provides optimum control or regulation of the brake system, but at a significant expense.
A modified individual control and regulation (MIC) anti-lock brake system is taught in German Patent DE-OS 28 51 107. As taught therein, this (MIC) anti-lock brake system provides improved control of the steering effort on roadways having variable or different force requirements on a roadway in which traction conditions change and differ from one side to the other. This improved steering control is accomplished by reducing the side drift or yawing moment of the vehicle during normal driving. For the so-called "high wheel", which is the wheel operating on the good side of the road, it has, for example, a pressure-holding or maintenance phase which is ensured if and when the so-called "low wheel" is operating on a bad side of the road and this (MIC) anti-lock brake system receives a pressure drop indication as a consequence of a deceleration and/or a wheel skid or slip signal.
In addition to individual control and regulation, it is also known in the prior art to utilize anti-lock brake regulation systems for an axle or a group of wheels. All such prior art systems can be used on a vehicle, regardless of whether the brake system is pneumatically operated or hydraulically operated. In the regulation system for an axle or group of wheels, one sensor is used for each wheel but, for cost consideration, a common setting or actuator unit is used for both wheels on an axle. For this reason, the brake cylinders of both wheels are connected in a parallel manner.
In a so-called select low control or regulation anti-lock brake system (SLC), that is frequently used on vehicles, the common brake pressure on both wheels is reduced and, thereafter, further regulated and controlled as a function of the movement of a particular wheel having the lower traction or lower power application (low wheel). In any event, a pressure reduction takes place in each prior art device as soon as a delay signal (-b) and/or a slip signal (.lambda.) is generated by the sensor associated with the low wheel. In such SLC anti-lock brake systems, certain low control settings will lead to a relatively high excess commencing force. In addition, this can, on occasion, lead to a significant amount of under-braking of the high wheel operating on a good side of the road. This then is a contributing factor to substantially extended braking distances encountered at times as compared with the (IC) anti-lock brake system, or even when braking with locked wheels. Such extended braking distances occur most frequently on so-called .mu.-split roadways, as well as on roadways with changing values for the coefficient of friction (such as icy conditions, etc.) or on uneven surfaces (such as cobblestone pavement, potholes, or generally poor roads) and with wheel brakes having different ratings.
In some cases, therefore, a so-called select high control or regulation (SHC) anti-lock brake system is used. Such SHC anti-lock brake system does not take into consideration the motional reaction in movement of the low wheel but, instead, reacts only to the retarding and/or slip signals generated from the sensor associated with the high wheel. Of course, the unavoidable result is a locking of the low wheel, even for rather long periods of time. Such a lockup of the low wheel (even if for an extended period of time) is, however, included with deliberate intent. Nevertheless, such (SHC) anti-lock brake system can lead to an undesirable flat tire condition at the low wheel, and to reduced conditions of vehicle stability when braking occurs on a curve on roadways having excellent traction conditions.
As far as such (SLC) or (SHC) anti-lock brake systems are used in commercial or utility vehicles, the pressure control and regulation in most cases is generally carried out by the use of a 3/2 way multi-directional solenoid valve. Such 3/2 way multi-directional solenoid valve has only a pressurization and evacuation phase, and not a pressure maintenance or holding phase.